1. Technical Field
The present invention relates to a discharge lamp lighting device, a control method thereof, and a projector.
2. Related Art
Discharge lamps such as high-pressure mercury lamps or metal halide lamps are used as a light source of projectors. The optimal driving conditions (a frequency, a duty ratio, and a waveform of driving current) of the discharge lamps depend on statuses of the discharge lamps. For example, just after starting the lighting and after a certain time passes since the lighting, the optimal driving conditions change. The optimal driving conditions are different in a discharge lamp having a short used time and a discharge lamp of which the lifetime almost expires. The optimal driving conditions change depending on the types of the discharge lamps.
The use of the discharge lamps under non-optimal driving conditions causes the blackening or devitrification of discharge tubes. It also causes a flicker. For example, when such discharge lamps are used in a projector, the brightness of a projected image changes in the course of using the projector.
Accordingly, a discharge lamp lighting device has been suggested which has a control circuit having plural driving conditions set in advance and which can properly select the driving conditions depending on a lighting status of a discharge lamp.
An example of such a known discharge lamp lighting device is disclosed in JP-A-2002-532866.
As described in JP-A-2002-532866, the lighting time of several hundreds hours to several thousand hours is required for detecting a continuous increasing phenomenon of a discharge lamp driving voltage. The negligence of the continuous increasing phenomenon of the discharge lamp driving voltage causes the brightness of the discharge lamp to be lowered. This is a problem particularly for an application intended to suppress the change in brightness of the discharge lamp as small as possible for a long time, such as a projector.
However, since a user usually uses the discharge lamps continuously for at most several hours, it is too short to detect the continuous increasing phenomenon of the discharge lamp driving voltage. Therefore, the controlling of the lighting conditions depending on the lighting status of the discharge lamp as in the discharge lamp lighting device described in JP-A-2002-532866 is not a satisfactory countermeasure.